1. Field of the Invention
The present invention relates to a planar waveguide device, and more particularly to an optical power splitter utilizing a Y-branch optical waveguide.
2. Description of the Related Art
An optical waveguide device includes an optical waveguide core through which an optical signal is transmitted by experiencing total internal reflection in the optical waveguide core, and a clad surrounding the optical waveguide core. Representative examples of optical waveguide devices include a planar waveguide device fabricated in a semiconductor manufacturing process and an optical fiber manufactured by melting an optical fiber preform. Planar waveguide devices include optical power splitter/couplers that divide or couple the power of the optical signal and wavelength division multiplexer/demultiplexers that multiplex or demultiplex channels constituting the optical signal according to wavelengths of the optical signal. Further, the optical power splitter may include a tapered waveguide, that enlarges field distribution of an input optical signal, and a Y-branch optical waveguide having a pair of output waveguide branches, that splits the enlarged optical signal and outputs it through ends of the output waveguide branches.
FIG. 1 is a schematic plan view of a conventional optical power splitter utilizing a Y-branch optical waveguide. The conventional optical power splitter 100 generally includes an input section 101, a branching section 102, and an output section 103. Further, the optical power splitter 100 is constructed so that both sides are symmetric with reference to an axis 230 of the optical power splitter 100.
The input section 101 includes an input waveguide section 110. The input waveguide section 110 receives an optical signal through its input end connected with an outer waveguide 250 and controls the device length of the optical power splitter 100.
In this case, in order to minimize the coupling loss of the optical signal, the input waveguide section 110 has a width 104 that can optimize the mode field diameter (MFD) of the outer waveguide 250 and the mode field diameter of the input waveguide section 110 and that enables single-mode propagation and low-loss insertion of the optical signal. That is, in order to minimize the coupling loss of the optical signal, the input waveguide section 110 enlarges the field distribution of the optical signal and functions as a coupling section that enables single-mode propagation and low-loss insertion of the optical signal. In this case, the outer waveguide 250 is a waveguide that constitutes an optical fiber or a planar waveguide device.
The branching section 102 includes a first tapered waveguide section 140, and first and second waveguide branches 150 and 160.
The first tapered waveguide section 140 receives the optical signal through its input end connected with the input waveguide section 110, and the width of the first tapered waveguide section 140 gradually increases in the direction toward which the optical signal propagates.
The first and second waveguide branches 150 and 160 extend from the output end of the first tapered waveguide section 140 symmetrically with reference to the axis 230.
The output section 103 includes second and third auxiliary waveguide sections 170 and 200, second and third tapered waveguide sections 180 and 210, and first and second output waveguide sections 190 and 220.
The second auxiliary waveguide section 170 controls the device length of the optical power splitter 100 and connects the first waveguide branch 150 and the second tapered waveguide section 180 with each other.
The second tapered waveguide section 180 receives a first branched optical signal through its input end connected with the second auxiliary waveguide section 170, and the width of the second tapered waveguide section 180 gradually increases in the direction toward which the first branched optical signal propagates.
The first output waveguide section 190 receives the first branched optical signal through its input end and outputs the first branched optical signal through its output end.
The third auxiliary waveguide section 200 controls the device length of the optical power splitter 100 and connects the second waveguide branch 160 to the third tapered waveguide section 210.
The third tapered waveguide section 210 receives a second branched optical signal through its input end connected with the third auxiliary waveguide section 200, and the width of the third tapered waveguide section 210 gradually increases in the direction toward which the second branched optical signal propagates.
The second output waveguide section 220 receives the second branched optical signal through its input end and outputs the second branched optical signal through its output end.
FIGS. 2A and 2B are graphs describing the mode matching in the case where an optical signal is input in alignment with the axis of the optical power splitter 100, that is, the case where the optical signal is input into the optical power splitter 100 in such a manner that the field distribution of the optical signal has a shape both sides of which are symmetric with reference to the axis 230.
The graph shown in FIG. 2A shows a first field distribution 310 in the input waveguide section 110 at the input end of the input waveguide section 110, which means a field distribution of the optical signal propagating through the input waveguide section 110 directly after passing the input end of the input waveguide section 110. As shown, the first field distribution 310 is arranged symmetrically with reference to the axis 230. This mode matching maximizes the coupling efficiency between the input waveguide section 110 and the optical signal.
The graph shown in FIG. 2B shows a third field distribution 320 in the first tapered waveguide section 140 and a fourth field distribution 330 in the first and second waveguide ranches 150 and 160, at the output end of the first tapered waveguide section 140. As shown, the first field distribution 310 and the fourth field distribution 330 are arranged symmetrically with reference to the axis 230. These mode matches maximize the coupling efficiency of the optical signal between the first tapered waveguide section 140 and the first and second waveguide branches 150 and 160.
FIG. 3 is a view showing an intensity distribution of the optical signal propagating through the optical power splitter 100 in the mode match state. In FIG. 3, sections 301, 302, and 303 represent the intensity distributions of the input section 101, branching section 102, and output section 103, respectively.
FIGS. 4A and 4B are graphs illustrating mode mismatch in the case where an optical signal is input in misalignment with the axis 230 of the optical power splitter 100. That is, these figures illustrate the case where the optical signal is input into the optical power splitter 100 in such a manner that the field distribution of the optical signal has a shape in which the sides are nonsymmetrical with reference to the axis 230.
The graph illustrated in FIG. 4A shows a fifth field distribution 340 in the input waveguide section 110 at the input end of the input waveguide section 110. As shown, a center line of the fifth field distribution 340 is not aligned with the axis 230. This mode mismatch degrades the coupling efficiency between the input waveguide section 110 and the optical signal.
The graph illustrated in FIG. 4B shows a sixth field distribution 350 in the first tapered waveguide section 140 and a seventh field distribution 360 in the first and second waveguide branches 150 and 160, at the input end of the first tapered waveguide section 140. As shown, the center lines of the sixth field distribution 350 and the seventh field distribution 360 are not aligned with each other. This mode mismatch degrades the coupling efficiency between the first tapered waveguide section 140 and the first and second waveguide branches 150 and 160.
FIG. 5 illustrates an intensity distribution of an optical signal propagating through the optical power splitter 100 in the mode mismatch state. In FIG. 5, sections 304, 305, and 306 represent the intensity distributions of the input section 101, branching section 102, and output section 103, respectively. As shown, the optical signal shakes while passing through the input section 101, thereby having an effect on the branching section B, which consequently causes the mode mismatch as shown in FIG. 4B.
FIG. 6 is a graph illustrating loss corresponding to the mode mismatch of the optical signal input to the optical power splitter 100, and FIG. 7 is a graph illustrating uniformity according to the mode mismatch of the optical signal input to the optical power splitter 100.
FIG. 6 illustrates a first output curve 370 representing the output of the first output waveguide section 190 according to the mode mismatch of the optical signal and a second output curve 380 representing the output of the second output waveguide section 220 according to the mode mismatch of the optical signal. As shown in FIG. 6, as the mode mismatch grows larger, the difference of the outputs between the first output curve 370 and the second output curve 380 becomes significantly larger.
FIG. 7 illustrates a uniformity curve 390 representing the difference between the outputs of the first and second output waveguide sections 190 and 220 according to the mode mismatch of the optical signal. As shown, the difference between the outputs of the first and second output waveguide sections 190 and 220 abruptly increases as the mode mismatch increases.
As described above, the conventional optical power splitter utilizing a Y-branch optical waveguide is problematic in that, the larger the mode mismatch, the larger the difference between the outputs of the first and second output waveguide sections 190 and 220. Moreover, the difference between the outputs of the first and second output waveguide sections 190 and 220 according to the mode mismatch of the optical signal further increases when the optical signal is a multi-channel signal.